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How can ReGen bypass the charger?

Discussion in 'Technical' started by emir-t, Sep 10, 2016.

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  1. emir-t

    emir-t Member

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    AFAIK when regening you create AC power from the AC motor something converts it into DC and puts it back into the battery. However batteries need a charger in between to get the current and voltage they want. It also takes a while for the battery to communicate with the EVSE when charging. With regen it is instantaneous. How does regen do that? When regenning the current you put in to the battery depends on your speed and how hard you're regenning.

    My EV for example only has a 4kW on board charger but I'm pretty sure it regens way above 4kW when slowing down. Can anyone explain how it works?
     
  2. Saghost

    Saghost Active Member

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    A car that has AC drive motors has a device called an inverter between the batteries and the motors which takes the battery power and converts it to the variable frequency AC the motor needs.

    When the car goes in to regeneration, the inverter changes the field timing resulting in the slowing effect, and the power flows 'backwards' through the inverter to produce battery voltage DC.

    The delays in your on board charger are due to the safety handshakes with the EVSE, and have nothing at all to do with regeneration, which does not involve the charger at all on any modern EV (some prototypes and conversions used the drive inverter as the onboard charger, but there are issues with that approach.)
     
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  3. emir-t

    emir-t Member

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    So the inverter makes the AC, DC in voltage batteries require. And the amperage doesn't matter because it is not ustained charging. Correct?
     
  4. siai47

    siai47 Member

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    As already stated, the inverter is bi-directional. It is output limited by design and software. In the Tesla designs, the inverter is limited in output during regeneration to a much lower level than it is capable off to make the deceleration caused by regeneration less severe and more controllable. Even if you could sustain maximum regeneration it is only about half of the power that a Supercharger inputs into the battery during a Supercharger session so no need to worry.
     
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  5. wycolo

    wycolo Active Member

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    #5 wycolo, Sep 10, 2016
    Last edited: Sep 10, 2016
    Imagine your Tesla is on a roller coaster. The climbing up part is easy to understand as all the battery energy being used is expended doing the rolling/lifting/heating. The heating is the electrical and mechanical losses. Going downhill the motor becomes a generator since it is being forced by gravity to turn faster than the accel pedal is requesting. We are assuming a prudent driver here, not someone just flooring the pedal like a maroon. To the extent that the motor is being turned faster than requested, that energy is being pumped seamlessly back into the battery.
    --
     
  6. emir-t

    emir-t Member

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    I know the basin principle of regen you are mentioning. I was asking matching voltages of energy from motor and battery. I guess because it is not going to be a sustained charge they don't care too much about the current going into the battery and just dump whatever is available.
     
  7. GSP

    GSP Member

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    The handshaking between an on-board charger and an EVSE is needed to insure the plug is physically connected before high voltage is activated, and for the charger to know the maximum safe current the EVSE and its supply wiring can provide.

    When the inverter switches to regeneration at closed throttle, all of the wiring is on-vehicle and already connected. The inverter software already is programmed with the max safe charging current for the battery and the max braking force for vehicle control, so it can start regen the moment the accelerator pedal is released.

    GSP
     
  8. wycolo

    wycolo Active Member

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    #8 wycolo, Sep 10, 2016
    Last edited: Sep 10, 2016
    It is a continuous dynamic process. An electric motor with no load spins at a fixed speed. If energy is added to this motor forcing the shaft to turn faster, that extra energy flows back into the source (battery) as it now is acting as a generator. And of course turning the shaft faster requires work. So the regen is there immediately without needing permission from software or control circuitry. If regen becomes excessive for the battery parameters at that given moment then circuitry will limit this excess.

    The spinning motor with no load is also a generator since it generates a 'back voltage'. The difference between applied voltage and back voltage in this case is proportional to the losses involved. But since there is no net generation being produced, we don't refer to it as being a generator.

    Such thoughts occur to me sometimes when entering my favorite 36 mile Rocky Mt downgrade and begin to see the milage figure jump up.
    --
     
  9. scaesare

    scaesare Active Member

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    To clarify: The car' current programming limits it to approximately half of a supercharger's power (~60kW).

    The inverter is capable of dealing with short term power levels in the 500+kW range for acceleration. We don't' know what it's theoretically capable of in regen mode, but it's maximum is likely far in excess of just 60kW.
     
  10. wycolo

    wycolo Active Member

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    If the devices in the inverter are possibly a bit asymmetrical nonetheless it should be able to carry serious amperage in both directions. Such that you might begin laying rubber as you enter a steep downhill section.
    --
     
  11. Jeff N

    Jeff N Active Member

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    Now if Tesla would just add a regen paddle to the steering wheel like the Chevy Bolt EV then the driver could safely regen higher than 60 kW without the need for friction braking using the brake pedal.... :)
     
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  12. scaesare

    scaesare Active Member

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    Hehe, I get the good natured jab...however I did start this thread 3 years ago:

    REGEN REQUEST
     
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  13. Panu

    Panu Member

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    Reading this thread I still did not understand why a charger is needed for AC charging. Couldn't it use the inverter instead?

    Reagarding regen paddle I have no idea why we would need a paddle when we already have the brake pedal.
     
  14. scaesare

    scaesare Active Member

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    Some designs do that (or a variation of it). As a matter of fact the early Roadster designs did so.

    However there are some tradeoffs/complexities. For one thing, the inverter typically handles multi-phase AC, whereas in many places the typical AC charging current is single-phase. There are other considerations as well.
     
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  15. Saghost

    Saghost Active Member

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    You don't want the car driving off while you're charging - which means that if you're going to use the drive inverter as a charger, you need a way to connect the inverter to the wall wires that isolates it from the motor coils and vice versa - which at a minimum means a set of big contactors between the motor and inverter and the inverter and charge port - actual moving parts to fail, and in the highest current areas of the EV.
     
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